Invasive tests can rule out or confirm a chromosome abnormality but carry a risk of miscarriage for up to three weeks after the test. In twin or multiple pregnancies invasive tests can show if each baby has normal chromosomes.
How long do test results take
Depending on the sample and the laboratory technique used, these tests usually provide an initial provisional result in days and a final confirmatory result within 3 weeks.
Chorionic Villus Sampling
CVS is performed from around 11 weeks of pregnancy onwards.A small sample of the placenta (afterbirth) is obtained, which can give us information about the baby because the baby and placenta developed from the same cell.Using ultrasound scan for guidance, a needle is inserted through the woman's abdomen to obtain a small sample from the placenta.Occasionally a sample from the placenta can be obtained through the cervix, in a similar manner to having a cervical smear. The exact cause of miscarriage following a CVS or amniocentesis is unknown. It is possibly due to ruptured amniotic membranes, infection or bleeding. The risk of miscarriage after CVS is between 1 and 2%, and the test result is slightly more likely to be inconclusive than amniocentesis. This risk is in addition to the natural background chance that a miscarriage may occur even if a test is not done.
Amniocentesis is performed from 15 weeks of pregnancy onwards. (Before this time there is not enough amniotic fluid to safely take a sample). Using ultrasound scan for guidance a needle is inserted through the woman's abdomen to obtain a small sample of amniotic fluid which contains cells from the baby The risk of miscarriage is about 1%.
This test is carried out after the 18th week of pregnancy and results take about a week. A needle is inserted into the mother's abdomen and a sample of the baby's blood is taken from the umbilical cord. The risk of miscarriage is between 1 and 4%.